1. Field of the Invention
The present invention relates to a process for preparing N, N-disubstituted urea and more particularly, to an improved process for preparing N,N'-disubstituted urea derivatives of formula (I) comprising ##STR2## wherein each of Ar.sup.1 and Ar.sup.2 represents unsubstituted aromatic radical or aromatic radical substituted with halogen atom, alkyl group, or alkoxy group, and Ar.sup.1 and Ar.sup.2 are the same or different, reacting aromatic mono-nitro compound, aromatic primary amines, and carbon monoxide in the presence of the catalyst composition consisting of a palladium compound as a main catalyst and an ammonium or a phosphonium salt containing halogen atom as a co-catalyst, and a non-polar solvent.
2. Description of the Prior Art
The conventional N,N'-disubstituted urea is an important intermediate for the production of carbamates which are raw materials for agrochemicals. The conventional methods for preparing N,N'-disubstituted urea have heretofore developed by reaction of amines with carbon monoxide in the presence of non-platinum catalysts such as cobalt carbonyl (Can. J. Chem., 40, 1718, 1962), silver acetate (J. Org. Chem., 37, 2670, 1972), and mercuric acetate; sulfur catalyst (J. Org. Chem., 26, 3309, 1961); and selenium catalyst (J. Am. Chem. Soc., 93, 6344, 1971).
Such methods for preparing N,N'-disubstituted urea in the presence of metal compounds except platinum group are not practical since the yield and selectivity of the reaction is quite low. Such methods using compounds such as sulfur catalyst, or selenium catalyst have high yield and selectivity. However, it is very difficult to separate and recover those catalysts. That is, unless the catalysts can be separated for reuse, the catalyst loss generally tends to make the expense of using the process prohibitive for economic purpose.
The processes using palladium group catalysts are disclosed in Japanese Patent Publication No. 53,41,123, Japanese Patent Laid Open Publication Nos. 58-144,363 and 62-59,253, and J. Org. Chem. Vol. 40 (19), 2819, 1975. Among the such disclosures, Japanese Patent Publication No. 53-41,123 and Japanese Patent Laid Open Publication No. 58-144,365 relate to the process fOr preparing N,N'-disubstituted urea by reaction of amines with carbon monoxide under an elevated temperature and a high pressure. In such methods, it is not only difficult to control the partial pressure of two kinds of gases involved, i.e. carbon monoxide and oxygen, but also there is a danger of explosion due to the oxygen. The method disclosed in J. Org. Chem. Vol. 40 (19), 2819, 1975 results in lower yield. Since tri-n-butyl amine is used together with solvent, the activity of the catalyst is suddenly decreased during the reaction. The process disclosed in Japanese Patent Laid Open Publication No. 62-59,253 gives relatively high yield and selectivity. However, it requires expensive catalysts such as rhodium and ruthenium compounds. Furthermore, the appearance of the resulting N,N'-disubstituted urea is not neat, and the used catalysts are unstable at a high temperature and decomposed around the reaction temperature.
The preparation of N,N'-disubstituted urea using a palladium compound as a catalyst is described in J. Org. Chem., 49 (19), 2819, 1975. Such method comprises the reaction of aromatic mono-nitro compound and aromatic primary amine with carbon monoxide in the presence of a palladium compound, an organic phosphine, tertiary amines, and tetraethyl ammonium chloride. The molar ratio of the aromatic mono-nitro compound to the aromatic primary amine is 1.1:1 to 2:1. Although it is ideal for this reaction to dissolve the palladium compound completely in the aromatic primary amine, it is actually very difficult to achieve. Also the reaction pressure is very low. As a result, the yield of the aromatic urea according to the method is as low as about 64%. Furthermore, when the product solids are separated from the reaction mixture after the reaction, a large amount of the catalyst mixed with the solid product is lost from the reaction mixture. In addition, a part of the palladium compound is decomposed. therefore, it is difficult to recover the catalyst efficiently and almost impossible to re-use the catalyst. Normally, the overall economy of the process depends on the efficiency of the recovery of expensive catalyst such as palladium.